Photosynthesis: The emergence of quantum biology

Despite decades of research, fundamental questions about photosynthesis remain unanswered. One important issue is whether we can use it to develop artificial systems capable of turning solar energy into carbon-neutral fuels (see “New sources of fuel“). The world has plenty of fossil fuels, but our unrestrained use of them will have severe consequences for the planet.

Progress here may require a step change in current approaches. Strange as it may seem, deciphering the quantum properties of the pigments involved may be the key we will need to master photosynthesis. Recent measurements have shown that when the pigments in light-harvesting antenna are excited by the energy of a photon, their electrons can jump into a quantum superposition of excited states. This “coherent” quantum state can last hundreds of femtoseconds or so.

Although this effect may seem subtle, it raises the intriguing possibility that such quantum states play a fundamental role in the early stages of the light reactions. It may also help to explain why photosynthetic antenna are so efficient at transferring light energy to the reaction centres.

Within any light-harvesting antenna complex, the protein structure will be changing constantly because of unavoidable thermal effects. These fluctuations cause the precise energies of the chlorophyll pigments bound to the protein to change, influencing the “energy landscape” of the system and either enhancing energy transfer processes or making them less efficient. However a coherent quantum state could, in principle, smooth out the effects of these fluctuations so that energy transfer always remains highly efficient.